The present invention relates to bonding of materials, and more particularly to brazeless bonding of dissimilar materials. Even more particularly, the present invention relates to brazeless hermetically sealed bonding of ceramic to metal for use in implantable devices.
stimulators that are to be implanted in living bodies and powered from external informational sources must be housed in packages of biocompatible material. Such packages must protect the electronic circuitry within the implanted stimulator from body fluids and ions so that the circuitry can survive for extended periods without any significant changes in performance.
Today, the most commonly used metals for implantable packages are titanium, stainless steel and cobalt-chromium alloys. These metals are biocompatible and corrosion resistant. Normally, the package consists of two parts welded together to insure hermeticity. The electrical components inside the package are connected to stimulating leads by hermetic feedthroughs, which permit the flow of electrical currents through the package while maintaining hermeticity. However, where there is a need to inductively couple an alternating electromagnetic field to an internal pickup coil, the metal package becomes a hinderance. Specifically, transmission of power is substantially reduced by eddy currents generated in the metal package due to the alternating electromagnetic field. To solve that problem, receiving coils are often placed outside the metal package, increasing the size and complexity of the of the implanted device.
It is known that the glasses and ceramics are transparent to alternating electromagnetic fields and that receiving antennas can be placed inside a hermetic zone of a ceramic or glass package, creating an overall smaller and simpler implant device and reducing the possibility of antenna failure due to saline leakage. Glasses and ceramics are inert and highly insoluble, which are favorable characteristics for long term implant materials. Unfortunately, however, because glasses and ceramics are inelastic, they are subject to fracture not only from mechanical shock but also from differential thermal expansion if even a moderate temperature gradient exists thereacross. Therefore, welding is not a practical method of sealing glass or ceramic materials. Instead, virtually the entire package and its contents must be raised to the melting temperature of the glass, ceramic or metal braze used to effect a sealing of the glass or ceramic package. Such sealing methods are unsatisfactory.
All known biocompatible glasses and ceramics are characterized by high sealing temperatures that will damage electronic components commonly included in electronic devices implanted in living bodies. Low melting temperature glasses all have the property of being corroded by body fluids. Further, metal or glass frits and solders useful in brazing glasses and ceramics and having melting temperatures below the thermal damage limits of implanted electronic components are either not biocompatible or corrode easily in body solutions. Therefore, packages composed entirely of ceramic and/or glass are not considered practical for such implant applications.
Also, in many ceramic and glass packages, the metal solder used to seal the main body and cap portions thereof forms a closed loop that is very close to coaxial with, or in a plane parallel to, the receiving coil used as the antenna for the electronics housed in the implantable package. Thus configured, the closed metal loop or solder acts as a shunt to the alternating electromagnetic fields impressed upon the package to transmit power and/or data to the implanted electronics. This has resulted in the generation of undesired heat within the package and the reduction of power transfer efficiency.
A packaged combination of one ceramic and two metal members is shown in U.S. Pat. No. 4,991,582, issued to Byers et al. and incorporated herein by reference. The one ceramic member is a ceramic case and one of the metal members is a metal band. The other metal member is a header plate. The ceramic case and the metal band are hermetically sealed together, each being characterized by similar coefficients of linear thermal expansion. The final package closure is effected by soldering the metal band to the ceramic case and the metal header plate to the metal band.
The junction between the ceramic case and metal band includes a bond of flat and smooth non-interlocking geometries. By such a design, forces resulting from unequal expansion or contraction of materials in or near the junction of the ceramic and metal members during temperature changes within and about the package are very inefficiently transferred to the ceramic members. This reduces the risk of residual strain and ultimately of fractures in the ceramic.
Alternatively, where the coefficients of linear thermal expansion of the ceramic case and metal band are similar, i.e., very close, the junction between the ceramic case and metal band may be interlocking to effect a self-jigging of the members during assembly. In such a form, temperature changes will produce corresponding changes in the geometries of the ceramic and metal members and undesired stresses on the junction will be minimized.
More particularly, the ceramic case shown in the '582 patent consists of a hollow flattened ceramic sleeve having a closed end and side walls and an open end for receiving electronic components of an implantable device, which are adversely sensitive to high temperatures such as those components that receive and transmit electromagnetic energy from or to the outside of the package. The coils comprising the antenna are positioned within the ceramic sleeve remote from and in a plane transverse and preferably normal to a flat annular end surface around the open end of the ceramic sleeve where the metal band is bonded. The metal band has a flat annular edge hermetically sealed as by a biocompatible metallic braze or glass solder to the flat annular end surface of the ceramic sleeve. Thus configured, the closed metal loop formed by the metal band and/or metal solder does not act as a shunt to power and/or information conveying alternating electromagnetic fields impressed upon the package and antenna of the present invention.
Finally, the header plate closes the package by means of an hermetic bond to the metal band. The header plate carries a plurality of electrical feedthrough connectors for connecting electrical leads to the electronic components within the package. The metal sleeve is bonded by high temperature welding, such as electron beam or laser welding, to the metal band after the electrical components are mounted in the ceramic sleeve (or case) and adequate heat sinking is applied to insure that there is no heat transfer to any heat sensitive electronic components or ceramic package component during the hermetic sealing operation.
Unfortunately, the package shown in the '582 patent still requires the use of a hermetically sealed weld or solder joint between the ceramic case and the metal band that suffers from one or more the following problems: (a) lack of biocompatability; (b) lack of corrosion resistance; (c) lack of electrolytic compatibility; (d) susceptibility to cracking of the ceramic case; and/or (e) toxicity. Thus, improvements are needed to overcome these problems with hermetically sealed bonds of ceramic to metal in packages for implantable devices.